1. Field of the Invention
The present invention relates generally to electromagnetic heating devices and, more specifically, to a heating head capable of heating a relatively large zone, particularly on curved surfaces.
2. Description of the Related Art
There currently exists a wide number and variety of induction heating devices. One such induction heating device is described in U.S. Pat. No. 4,521,659, issued to Buckley et al. on Jun. 4, 1985. The heating gun described therein uses a tank circuit which includes a capacitor and an inductor coil. The inductor coil is wrapped around a U-shaped pole piece which has a gap formed between the ends thereof. The capacitor of the tank circuit is connected to a power supply which provides an alternating current of predetermined frequency.
The aforementioned patent describes a technique of joining two sheets of material by placing a susceptor, such as a wire screen, between the sheets with adhesive therebetween. The heating gun is positioned above the two sheets and the screen with the ends of the pole piece touching one of the sheets and with a gap of the pole piece located above the area where the sheets are to be joined. An alternating current from the power source enters the tank circuit and the capacitor increases the Q of the tank circuit, in turn increasing the current flow through the inductor coil. Direction of the current along the inductor coil reverses at each cycle of alternating current. The current flow in the inductor coil creates a magnetic flux within the turns of the coil. The flux is picked up by the pole piece and carried to either of the two ends. The flux then jumps to the susceptor rather than across the gap to the other of the two ends. Since the sheets of material are transparent to magnetic flux, the flux is easily transferred to the screen through the sheets. The rapidly changing direction of current in the coil causes the flux to change constantly in magnitude and direction. This is also true in the metal screen as well as in the pole piece. Hysteresis creates eddy currents in the screen which result in heat being generated in the screen.
While the aforementioned heating gun is suitable for laying down a single heat zone, whereby a seam can be created between two pieces of layered material, particularly for flat surfaces, it is sometimes desirable to heat curved surfaces to form multi-ply composites.
Prior methods have employed induction heating of high reluctance metal screens or perforated metal sheets sandwiched between two pieces of thermoplastic or screen susceptors or metal sheet susceptors encapsulated with adhesives sandwiched between non-thermoplastic adherences. These methods generally require the use of a susceptor in the bondline which, in some bonding processes, is considered unacceptable because of possible corrosion or galvanic action. With conductive susceptors, it is difficult to use conventional non-destructive evaluation of joints and seam welds or bonds. In the past, electric heat blankets have been used to process adhesives in bondlines. Heat blankets are limited to process temperatures of about 450.degree. F., however.
Additionally, the use of induction heating devices for patch repair of damaged aircraft structures, such as wind screens, wing surfaces, etc., normally require vacuum bagging. Vacuum bagging assures proper pressure between patches and the damaged surface area while maintaining patch stability during the bonding process. Typically, vacuum bagging removes excessive air from the bondline/adhesive resulting in a higher strength repair.
Vacuum bagging typically includes applying a ring or layer of sealant, vacuum tape, to surfaces being repaired. A suction end of a vacuum hose is positioned inside the sealant ring and held in place by additional sealant at the point where the hose crosses the sealant ring. A layer of material such as Kypton film is placed over and in contact with the sealant ring. A vacuum is created in the patch area when suction is applied to the vacuum hose. In conventional vacuum bagging, as described above, the process is time consuming and requires a variety of materials. Moreover, it is extremely difficult where overhead patches are required or when working in harsh weather conditions.
The heating head itself is an important consideration. Previous induction heating devices utilize heating heads with rigid coil core material for focusing the heat-generating flux to a specific area. This is unacceptable when trying to apply the heat to a curved surface since the core material would prevent the coil from conforming to the shape of the surface. Since the flux transferred from the coil to any point on the susceptor is a function of the distance between that point and the coil, uneven heating of the surface will result.